1. Field of the Invention
This invention relates to a charge transfer device used for solid state image pick-up devices, and the delay lines, comb type filters and transversal filters of the charge transfer type, and more particularly to a charge transfer device with an improved output circuit which is for converting a signal charge into a signal voltage.
2. Description of the Related Art
Recently, charge transfer devices have widely been used in video devices, such as televisions, video disk players (VDPs), and video tape recorders (VTRs), due to their excellent broad band characteristics and delay characteristics. One of the known signal outputting systems used as the output circuit of the CTD is a floating diffusion system. FIG. 1 shows a circuit arrangement of a conventional CTD based on the floating diffusion system. In the figure, reference numerals 61 to 64 show respectively transfer electrodes provided in a charge transfer section. For sequentially transferring signal charges in this section, a pair of electrodes 61 and 62 are driven by transfer pulse .phi.1, and another pair of electrodes 63 and 64 are driven by another transfer pulse .phi.2. The signal charges are transferred through the semiconductor region under these electrodes 61 to 64, and then pass through output gate electrode 65 and enter floating diffusion region 66. Floating diffusion region 66 is connected to output circuit 69 of the source follower type, which is made up of enhancement type (E type) MOS transistor 67 and power source 68. Output circuit 69 converts the signal charge into a signal voltage VO, and outputs it as an output signal. After the voltage signal is output from output circuit 69, reset gate electrode 70, which is provided adjacent to floating diffusion region 66 and is under control of reset pulse .phi.R, is enabled, so that the signal charges are discharged as unnecessary charges into drain region 71.
In recent portable VTRs there is a tendency to use a low voltage, e.g., 5 V, for the power source voltage in the circuit system. This tendency demands that the CTDs be operable at low voltage. However, the output circuit of the CTD must operate with good linearity.
To maintain good linearity of output circuit 69, transistor 67 must operate in a saturation region. To this end, the following relation shall be satisfied EQU VGG-.DELTA.VFD+.DELTA.V-VTH&lt;VDD, (1)
where
VGG: reset voltage applied to drain region 71, PA1 VDD: power source voltage of output circuit 69, PA1 .DELTA.VFD: DC bias component of the signal charge transferred to floating diffusion region 66, PA1 2.DELTA.V: AC signal component of the charges to be transferred to said floating diffusion region, PA1 VTH: threshold voltage of drive MOS transistor 67.
An attempt to realize a floating diffusion type CTD operable at a low voltage, will encounter the following problem. The amount of charge contained in the output section including floating diffusion region 66 and drain region 71, is reduced, so that the dynamic range of the CTD is narrowed. To cope with this, a pulled-up high voltage relative to power source voltage VDD is used for the reset voltage VGG applied to drain region 71.
If such a pulled-up voltage is used for reset voltage VGG, it is difficult to satisfy the relation (1). Hence, transistor 67 operates in a nonsaturation region and the CTD output circuit operates nonlinearly.
As described above, in the conventional CTD, if attempt is made to reduce the power source voltage with a satistactory dynamic range, the linearity of the output circuit is damaged.